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Home page > Training > PhD and internships (M2)

M2 - Scattered light studies in the detection system of the Advanced Virgo+ interferometer.

The Virgo gravitational wave detector, installed in Pisa, works on the principle of a laser interferometer with 3 km long arms. In 2017, the first joint observation of gravitational waves, detected by the LIGO and Virgo detectors, from the coalescence of a binary neutron stars system, and a gamma-ray burst, followed by the observation of an optical signal, opened a whole new chapter in multi-messenger astronomy. A period of joint observation of LIGO (in the United States) and Virgo detectors followed from April 2019 to March 2020, allowing the weekly detection of coalescences of compact binary systems (notably black holes). Since 2020, the Virgo detector has entered a phase of modifications and improvements (upgrade Advanced Virgo +) with the goal of entering a new observation period in Spring 2023.

Several optical benches, suspended and placed under vacuum, are used to extract the laser beam in different locations (at the output of the interferometer, but also in transmission of the kilometer arms, for example). These benches house optical elements and light sensors (photodiodes, quadrant photodiodes, cameras, etc.) which have the role of extracting the signals useful for the detection of gravitational waves and for the control of the position of the interferometer mirrors. The dark fringe beam obtained at the output of the interferometer is also filtered by an optical cavity, called mode cleaner, which has the role of eliminating the parasitic components of the beam which do not participate in the detection of gravitational waves: beams not having the wavelength or not the geometry of the main beam.

The mode cleaner cavity as well as the other optical elements diffuse part of the incident light. This diffused light can recombine with the main beam, and, by interference, generate variations in the laser power measured on the main photodiodes, variations which are interpreted as the passage of a gravitational wave. In addition, this scattered light also couples by modulating the radiation pressure of the laser beam on the interferometer mirrors, generating a real variation in differential length of the interferometer arms. By these various couplings, this scattered light therefore limits the sensitivity of the detector. In the context of the Advanced Vigo + upgrade, a review of the optical benches is undertaken in order to identify mitigation actions to reduce the scattered light and thus improve the sensitivity of the detector. Moreover, it is envisioned in the longer term to change the design of the mode cleaner cavity in order to improve its performances, particularly in terms of optical losses. These optical losses are largely induced by the light scattered on the reflecting surfaces and by Rayleigh scattering in the cavity substrate. It is therefore essential to characterize the scattered light emitted by the mode cleaner cavity.

LAPP offers a Master 2 internship in order to participate in the characterization of the light scattered on the suspended optical benches of the Virgo interferometer. To this purpose, the intern student will be asked to characterise the light backscattered by optical components used in Virgo, such as a mode cleaner cavity, or a Terbium Gallium Garnet crystal used in Faraday isolators. The student will use an experimental setup to measure by interferometry the light backscattered by the tested samples. The student will have to take control of the measurement setup and to adapt it to the specific needs of the components to be characterised. In particular, the mode cleaner cavity must be kept resonant during the measurement by means of feedback loops. The measurements obtained during the internship will then be compared with the projections of the scattered light noise in the Virgo interferometer, in order to identify the main sources of scattered light.

Optional information on a possible PhD subject:

This internship can lead to a PhD subject encompassing the problem of scattered light in the detection system, the study of its effect on the Virgo sensitivity and, more generally, the study and improvement of the sensitivity of the Virgo detector during the next data-taking run O4 (2023-2024), and for the future data-taking run O5 (start of installation foreseen in 2024). The thesis subject could also focus on the development of a lower loss mode cleaner cavity for future Virgo upgrades.

o Level : Master 2 internship, can be followed by a PhD

o Date of the internship : spring 2023

o Supervisor : Michal Was, Romain Gouaty

o Phone : 04 50 09 55 19

o e-mail :